The invention relates generally to wireless communications. More particularly, the invention relates to scheduling and wireless transmission of data between multiple base transceiver stations and subscriber units, providing spatial multiplexing and communication diversity.
Wireless communication systems commonly include information carrying modulated carrier signals that are wirelessly transmitted from a transmission source (for example, a base transceiver station) to one or more receivers (for example, subscriber units) within an area or region.
Spatial Multiplexing
Spatial multiplexing is a transmission technology that exploits multiple antennae at both the base transceiver station and at the subscriber units to increase the bit rate in a wireless radio link with no additional power or bandwidth consumption. Under certain conditions, spatial multiplexing offers a linear increase in spectrum efficiency with the number of antennae. For example, if three antennae are used at the transmitter (base transceiver station) and the receiver (subscriber unit), the stream of possibly coded information symbols is split into three independent substreams. These substreams occupy the same channel of a multiple access protocol. Possible same channel multiple access protocols include a same time slot in a time-division multiple access protocol, a same frequency slot in frequency-division multiple access protocol, a same code sequence in code-division multiple access protocol or a same spatial target location in space-division multiple access protocol. The substreams are applied separately to the transmit antennae and transmitted through a radio channel. Due to the presence of various scattering objects in the environment, each signal experiences multipath propagation.
The composite signals resulting from the transmission are finally captured by an array of receiving antennae with random phase and amplitudes. At the receiver array, a spatial signature of each of the received signals is estimated. Based on the spatial signatures, a signal processing technique is applied to separate the signals, recovering the original substreams.
FIG. 1 shows three transmitter antenna arrays 110, 120, 130 that transmit data symbols to a receiver antenna array 140. Each transmitter antenna array includes spatially separate antennae. A receiver connected to the receiver antenna array 140 separates the received signals.
FIG. 2 shows modulated carrier signals traveling from a transmitter 210 to a receiver 220 following many different (multiple) transmission paths.
Multipath can include a composition of a primary signal plus duplicate or echoed images caused by reflections of signals off objects between the transmitter and receiver. The receiver may receive the primary signal sent by the transmitter, but also receives secondary signals that are reflected off objects located in the signal path. The reflected signals arrive at the receiver later than the primary signal. Due to this misalignment, the multipath signals can cause intersymbol interference or distortion of the received signal.
The actual received signal can include a combination of a primary and several reflected signals. Because the distance traveled by the original signal is shorter than the reflected signals, the signals are received at different times. The time difference between the first received and the last received signal is called the delay spread and can be as great as several micro-seconds.
The multiple paths traveled by the modulated carrier signal typically results in fading of the modulated carrier signal. Fading causes the modulated carrier signal to attenuate in amplitude when multiple paths subtractively combine.
Communication Diversity
Antenna diversity is a technique used in multiple antenna-based communication system to reduce the effects of multi-path fading. Antenna diversity can be obtained by providing a transmitter and/or a receiver with two or more antennae. These multiple antennae imply multiple channels that suffer from fading in a statistically independent manner. Therefore, when one channel is fading due to the destructive effects of multi-path interference, another of the channels is unlikely to be suffering from fading simultaneously. By virtue of the redundancy provided by these independent channels, a receiver can often reduce the detrimental effects of fading.
An individual transmission link exists between each individual base transceiver station antenna and a subscriber unit in communication with the base transceiver station. The previously described spatial multiplexing and communication diversity require multiple antennas to each have transmission links with a single subscriber unit. Optimally, the base transceiver station can schedule data transmission according to the transmission link quality.
It is desirable to have an apparatus and method that provides scheduling of transmission of data blocks between multiple base station transceivers and receivers (subscriber) units. It is desirable that the scheduling be adaptive to the quality of transmission links between the base station transceivers and the receivers (subscriber) units. It is additionally desirable that the apparatus and method allow for spatial multiplexing and communication diversity through the multiple base station transceivers.
The invention includes an apparatus and a method for scheduling wireless transmission of data blocks between multiple base transceiver stations and multiple receiver (subscriber) units. The scheduling can be based on the quality of a transmission link between the base transceiver stations and the receiver units, the amount of data requested by the receiver units, and/or the type of data requested by the receiver units. The scheduling generally includes assigning frequency blocks and time slots to each of the receiver units for receiving or transmitting data blocks. The transmission scheduling allows for spatial multiplexing and communication diversity through the multiple base station transceivers.
A first embodiment of the invention includes a wireless communication system. The wireless communication system includes a scheduler unit. The scheduler unit receives the protocol data units from a network and sub-divides the protocol data units into sub-protocol data units. A plurality of base transceiver stations receive the sub-protocol data units, and wirelessly transmit the sub-protocol data units to a subscriber unit. The scheduler unit determines a schedule protocol for transmission of the sub-protocol data units by the plurality of base transceiver stations.
A second embodiment of the invention is similar to the first embodiment. The second embodiment further includes base controller station. The base controller station includes the scheduler unit.
A third embodiment is similar to the second embodiment. The third embodiment includes a standard network interconnection for providing a sub-protocol data units transfer path between the base controller station and the base transceiver stations.
A fourth embodiment is similar to the third embodiment. The third embodiment includes the sub-protocol data units being encapsulated within standard data units that corresponds to the standard network interconnection. The standard network connection can be an ATM network connection and the standard data units can be ATM cells. Alternatively, the standard network connection can be an IP network connection and the standard data units can be IP cells.
A fifth embodiment is similar to the first embodiment. The fifth embodiment includes the plurality of base transceiver stations including a home base transceiver station. The home base transceiver station includes the scheduler unit. The home base transceiver station can be the base transceiver station that has a highest quality transmission link with the subscriber unit.
A sixth embodiment is similar to the fifth embodiment. The sixth embodiment includes a standard network interconnection for providing a sub-protocol data units transfer path between the home base transceiver station and the base transceiver stations. The sub-protocol data units can be encapsulated within standard data units that corresponds to the standard network interconnection. The standard network connection can be an ATM network connection and the standard data units can be ATM cells. Alternatively, the standard network connection can be an IP network connection and the standard data units can be IP cells.
A seventh embodiment is similar to the first embodiment. The seventh embodiment includes the sub-protocol data units being transmitted between the base transceiver stations and the subscriber unit in data blocks, in which the data blocks are defined by a frequency block and time slot. A number of sub-protocol data units that are within a data block can be dependent upon a quality of transmission links between the base transceiver stations and the subscriber unit.
An eighth embodiment includes a method of transmitting within a cellular wireless system. The method includes receiving protocol data units from a network, sub-dividing the protocol data units into sub-protocol data units, scheduling transmission of the sub-protocol data units from a plurality of base transceiver stations to a subscriber unit and transmitting the sub-protocol data units according to the scheduling. The sub-dividing and scheduling can occur within a base controller station. The sub-protocol data units can be transferred from the base controller station to the base transceiver stations by encapsulating the sub-protocol data units within a standard network interconnection protocol cell.
An ninth embodiment includes a method for transmitting data streams between a plurality of base transceiver stations and a subscriber unit. The method includes receiving data requests from the subscriber unit, and once per frame of time, generating a schedule based on the data requests, that designates time slots and pre-defined frequency blocks in which the subscriber is to transmit sub-protocol data units to the plurality of base transceiver stations.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.